I was trying to send some photos from home to work (Auckland to Auckland) yesterday but the photos kept failing to load. I had a look in the Details link on Gmail (bottom right of Gmail screen). According to that I was accessing Taiwan for my mail: 'This computer is using IP address 2001:c08:3700:ffff::9009. (Taiwan)'.

Then I remembered I was using an IPV4/IPV6 tunnel (thank you Timmmay). Checking that showed that it was using a server in Taipei. So I changed to one in Montreal and it seems to have fixed the problem. Now Gmail connects locally. Also starts and closes down a lot quicker. It had become quite slow recently.